US Defense Secretary Jim Mattis reportedly warned Sweden of severe consequences if the country followed through on signing a UN treaty banning nuclear weapons.

The Scandinavian country is one of 122 states backing the treaty, and Stockholm also recently signed a statement of intent to increase military cooperation with the US.

But a letter from Mattis reportedly warned Sweden's defense minister, Peter Hultqvist, that signing on to the treaty could affect US-Sweden military cooperation as well as US military support in the event of war.

Mattis' letter also suggested signing the treaty could have an impact on the country's ties to NATO, of which it is a Gold Card program member, meaning it has some privileges within the defense alliance even though it is not a full member.

Sweden's Gold Card program status faces renewal in October, and Mattis warned his Swedish counterpart that signing the treaty would foreclose the option of joining NATO, according to Defense News.

Swedish newspaper Dagens Nyheterin also cited a source as being concerned the threat could apply to US-Sweden defense-industry cooperation, including deals of which Saab is a part. (The Swedish government recently completed a cross-party deal to boost domestic defense spending.)

Swedish Foreign Minister Margot Wallström has said the country intends to sign on to the treaty, though Hultqvist is reportedly against doing so.

The US, which adheres to a policy of nuclear deterrence, has criticized the nuclear-weapons ban, but Mattis' letter is seen as an unusual step in bilateral relations, particularly between the US and Sweden.

A Pentagon spokesman told Defense News that while the US "values its defense relationship with Sweden," it has discouraged countries from signing on to the ban, which has measures that "could potentially affect our ability to cooperate with parties to the treaty on issues of mutual interest."

"The government's attitude towards these weapons is well known since long," Wallström told local news outlet Svenska Dagbladet.

Jim Townsend, who was deputy assistant secretary of defense for European and NATO policy for eight years, told Defense News that pressuring Sweden with threats about defense cooperation is a flawed approach.

A 160-man combat team from the Skaraborg Armored Regiment trains on the island of Gotland in Visby harbor, Sweden, September 14, 2016. Soren Andersson/TT News Agency via REUTERS

"They are a close friend in a dangerous neighborhood, and so threatening that important relationship lacks some credibility," said Townsend, who is now with the Center for New American Security. "Do the Swedes really think we would downgrade our relationship to punish them for signing a nuclear ban treaty?"

Townsend also stressed caution when using these tactics in important bilateral relationships. "The cause had better be worth the risk" to US national security and to relations between the two countries, he told Defense News.

Sweden and its neighbors, Finland and Norway, maintain military neutrality, but they work closely with the US and allies in Europe on military matters. Those relationships have grown in importance amid escalating tensions between Russia and countries in Europe.

President Donald Trump with Finnish President Sauli Niinisto at the White House, August 28, 2017. Thomson Reuters

Norway, which is looking to boost its own border defenses, has played host to US military equipment for decades and recently welcomed an extended deployment of US Marines.

It's the first time a foreign force had been posted on Norwegian soil since World War II, which has irked Russia.

During an appearance with Finnish President Sauli Niinistro in late August, Trump falsely claimed that Helsinki was planning to buy F/A-18 fighter aircraft from US defense firm Boeing.

While Finland is looking to buy new fighter aircraft, it has not made a deal with Boeing to do so. Trump's comments prompted a denial from Niinistro afterward.

"It seems that on the sale side, past decisions and hopes about future decisions have mixed," he said.